This invention relates to the fabrication of gratings on the surface of a body.
In recent years a vast number of devices employing grating relief patterns (i.e., arrays of parallel stripes separated by grooves) on surfaces have been proposed or demonstrated. These devices include distributed feedback heterostructure junction lasers, optical grating couplers and optical frequency filters. For many of these applications, it is important to be able to specify and control not only the grating period, but also the depth and shape of the grating profile. For example, the shape of the stripes in a grating coupler determines the distribution of diffracted light between two possible directions. The depth of the grooves in a distributed feedback structure, on the other hand, determines the reflectivity of the grating. In the prior art, these gratings are generally made by masking the body with photoresist stripes and then ion-milling. This procedure is adequate as long as the milling rate of the body exceeds that of the photoresist by a significant amount. Otherwise, the photoresist will be milled away before any appreciable grating depth is achieved, as with the case of YAG which has a milling rate less than half that of typical photoresists. Aside from the relative milling rates of the mask and body, however, prior art grating fabrication technique is generally not amenable to shaping the grating profile to satisfy specialized functions such as required of the optical grating coupler.